Can I set boundaries?

My sister is experiencing psychosis, and during the periods where she is in a slightly better mood, I have very positive interactions with her. Lately, though, she has become incredibly hostile, and it is getting harder for me to deal with, given that there are no guidelines as to how I'm supposed to interact with her. When she yells at me to do something, it is wise for me to calmly tell her I don't like that and that I will do the thing once she has asked me nicely? Is it better for me to just do whatever she says, no questions asked? Or should I stop talking to her altogether? I honestly don't know. Advice?